WASHINGTON (Reuters) - A Somali man who was a high-level liaison between al Qaeda affiliates in Somalia and Yemen and later became a U.S. government informant and witness has pleaded guilty to multiple U.S. terrorism-related charges, the Justice Department said on Monday.

Federal prosecutors in Manhattan unsealed a guilty plea by Ahmed Abdulkadir Warsame to nine U.S. criminal charges. They alleged he once commanded hundreds of fighters for the Somalia-based militant group Al-Shabaab.

Law enforcement and legal sources said Warsame is one of the most important militants connected to Somali-based al Qaeda affiliate Al-Shabaab and Yemen-based Al Qaeda in the Arabian Peninsula (AQAP) to be captured by U.S. forces and to agree to become a U.S. informant.

Information provided to U.S. authorities by Warsame has resulted in the capture or arrest of other suspected al Qaeda militants from East Africa, lawyers familiar with recent prosecutions brought by U.S. authorities said.

In one of those cases, former al Shabaab members detained in the African state of Djibouti last year were brought to New York for trial in U.S. courts without any known extradition proceedings in Djibouti, the lawyers said.

Critics of U.S. counter-terrorism policy alleged that the case was an example of how the administration of President Barack Obama has continued to engage in "rendition," a procedure under which suspected militants are moved from country to country without normal legal procedures.

While it has foresworn some controversial counter-terrorism practices used by the administration of President George W. Bush, including the use of physically coercive interrogation tactics and secret CIA prisons, the Obama administration announced it would not completely abandon the use of rendition.

According to one official document unsealed on Monday with Warsame's guilty plea, U.S. authorities seized a laptop and two other electronic devices when he was arrested two years ago. The memories of these devices, prosecutors said, contained "dozens of pages of handwritten notes by Warsame as he learned how to build bombs; letters back and forth between the senior leaders of AQAP and al Shabaab, and correspondence in which Warsame describes why he was sent to Yemen by al Shabaab."

In the same document, prosecutors said that information provided to the United States by Warsame was corroborated by at least four other witnesses who interacted extensively with him in Africa, three of whom are presently cooperating witnesses in a Shabaab-related case in Minnesota, which has a substantial Somali-American community.

REDUCED SENTENCE?

Prosecutors said that Warsame's guilty plea was made in secret on December 21, 2011. But the records were sealed as a result of what the government described as a "cooperation agreement" between Warsame and U.S. authorities.

Although the maximum sentence Warsame could receive for his guilty plea is life in prison, customarily prosecutors agree to request a reduction in sentence for suspects who cooperate extensively with investigators.

A law enforcement official said the case had been unsealed because investigators felt that keeping it secret would no longer potentially compromise ongoing investigations. It was unclear when Warsame will be sentenced.

A Justice Department official said that Warsame had begun cooperating with U.S. authorities after he was captured by U.S. military forces in April 2011. For the next two months, the official said, he was questioned "for intelligence purposes."

A law enforcement official said these interrogations were conducted while Warsame was held aboard a U.S. military ship sailing off the coast of Africa.

Subsequently, the Justice Department said in a press release, Warsame was read his Miranda rights. But after waiving those rights, he continued to cooperate with U.S. investigators, the department said.

A Justice Department official said that U.S. authorities continue to make "active use" of information provided by Warsame and that his cooperation "has been and continues to be enormously valuable." Warsame remains in custody in the United States.

Prosecutors alleged in unsealed documents that, in addition to leading and training Shabaab fighters, Warsame also served as a liaison between the Somalia-based group and AQAP, which U.S. authorities consider to be perhaps Al Qaeda's most dangerous affiliate.

The documents say that after waiving his rights on around seven separate occasions, Warsame "confessed to agents" that he had fought alongside and commanded "hundreds of al Shabaab fighters in battle in Somalia."

In late 2009, the documents say, al Shabaab leaders sent Warsame to meet and train with AQAP in Yemen. There, U.S. authorities allege, he received "military, explosives and weapons training from AQAP, assisted in the exchange of communications between senior members of al Shabaab and AQAP," and allegedly facilitated al Shabaab weapons purchases from AQAP.

Among the charges to which Warsame pleaded guilty were providing material support to both Al Shabaab and AQAP, as well as conspiring to teach and demonstrate the making of explosives and possessing firearms and explosives including machine guns, the Justice Department said.